Myocardial infarction as a problem of growth control: cell cycle therapy for cardiac myocytes?

Targeted MicroRNA Interference Promotes Postnatal Cardiac Cell Cycle Re-Entry

Mammalian heart cells undergo a marked reduction in proliferative activity shortly after birth, and thereafter grow predominantly by hypertrophy. Our understanding of the molecular mechanisms underlying cardiac maturation and senescence is based largely on studies at the whole-heart level. Here, we investigate the molecular basis of the acquired quiescence of purified neonatal and adult cardiomyocytes, and use microRNA interference as a novel strategy to promote cardiomyocyte cell cycle re-entry. Expression of cyclins and cyclin-dependent kinases (CDKs) and positive modulators were down-regulated, while CDK inhibitors and negative cell cycle modulators were up-regulated during postnatal maturation of cardiomyocytes. The expression pattern of microRNAs also changed dramatically, including increases in miR-29a, miR-30a and miR-141. Treatment of neonatal cardiomyocytes with miRNA inhibitors anti-miR-29a, anti-miR-30a, and antimiR-141 resulted in more cycling cells and enhanced expression of Cyclin A2 (CCNA2). Thus, targeted microRNA interference can reactivate postnatal cardiomyocyte proliferation.

Concise review: stem cells, myocardial regeneration, and methodological artifacts

This review discusses the current controversy about the role that endogenous and exogenous progenitor cells have in cardiac homeostasis and myocardial regeneration following injury. Although great enthusiasm was created by the possibility of reconstituting the damaged heart, the opponents of this new concept of cardiac biology have interpreted most of the findings supporting this possibility as the product of technical artifacts. This article challenges this established, static view of cardiac growth and favors the notion that the mammalian heart has the inherent ability to replace its cardiomyocytes through the activation of a pool of resident primitive cells or the administration of hematopoietic stem cells.

Life and death of a cardiac myocyte: principles of cellular biology

If the future of extracorporeal circulation is to include approaches to enhance localized or widespread distribution of cells, and/or gene transfer for augmentation of cardiac function, it is imperative that we gain an increased understanding of the mechanisms that define the cardiac myocyte phenotype. The purpose of this paper is to review the natural history of the cardiac myocyte. A variety of signals determine the cellular processes that characterize birth, growth, differentiation and death of cardiomyocytes. Examined here are primary aspects of the molecular genetics of growth and development, including signal transduction, protein phosphorylation, the cell division cycle, and transcriptional activation. This review is intended to be an update on insights into molecular aspects of the cell, with emphasis on gene expression during cardiac myogenesis and a discussion of its relevance to the field of extracorporeal circulation. In addition, the current status of research in myogenesis is presented.

Myocardial regeneration: present and future trends

Cardiomyocytes are terminally differentiated and are unable to proliferate in response to injury. Genetic modulation, cell transplantation and tissue engineering promise a revolutionary approach for myocardial regeneration and tissue repair after myocardial injury. Current data derived from animal models suggest that it may be possible to treat heart failure by inserting genetic materials or myogenic cells into injured myocardium. Success with animal models has raised the hope for new treatment after heart attacks and could prove an alternative to transplantation, particularly in elderly patients for whom there is often a lack of donor hearts. This exciting research, however, still faces significant difficulties before it can develop into a clinical therapeutic tool and many challenges need to be overcome before cell transplantation, gene therapy and tissue engineering can be considered efficient, therapeutic strategies for myocardial regeneration.

Phenylephrine protects neonatal rat cardiomyocytes from hypoxia and serum deprivation-induced apoptosis

Previous studies have shown that alpha-adrenergic activation reduces myocardial damages caused by ischemia/reperfusion. However, the molecular mechanisms of how alpha-adrenergic activation protects the myocardium are not completely understood. The objective of this study was to test the hypothesis that alpha-adrenergic activation protects the myocardium by, at least in part, inhibiting apoptosis in cardiomyocytes. The current data has shown that apoptosis in neonatal rat cardiomyocytes, induced by 24 h treatment with hypoxia (95% N2 and 5% CO2) and serum deprivation, was inhibited by co-treatment with phenylephrine. Pre-treatment with phenylephrine for 24 h also protected cardiomyocytes against subsequent 24 h treatment with hypoxia and serum deprivation. Exposure of cardiomyocytes to phenylephrine for up to 9 days under normoxic conditions did not cause apoptosis. The phenylephrine-mediated cytoprotection was blocked by an alpha-adrenergic antagonist, phentolamine. beta-adrenergic activation with isoproterenol did not protect cardiomyocytes against hypoxia and serum deprivation-induced apoptosis. Under hypoxic conditions, phenylephrine prevented the down-regulation of Bcl-2 and Bcl-X mRNA/protein and induced hypertrophic growth. Phenylephrine-mediated protection was abrogated by the phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor wortmannin and was mimicked by the caspase-9 peptidic inhibitor LEHD-fmk. These results suggest that alpha-adrenergic activation protects cardiomyocytes against hypoxia and serum deprivation-induced apoptosis through regulating the expression of mitochondrion-associated apoptosis regulatory genes, preventing activation of mitochondrial damage-induced apoptosis pathway (cytochrome C-caspase-9), and activating hypertrophic growth.